1. Field of the Invention
The present invention relates to welding methods used in tungsten-inert-gas (TIG) welding of steel-based materials.
2. Description of Related Art
Conventionally, structures made of steel-based materials, such as carbon steel or stainless steel as the base material, are welded using TIG welding processes. TIG welding is widely used as a method of welding structures requiring high reliability since the welding operation can be easily performed and the welded metal portion can be formed with high-quality (See Japanese Patent Application Laid Open No. 2003-019561). However, since steel-based materials, including stainless steel based materials used in recent years usually contain less sulfur as an impurity, the welded metal portion formed during TIG welding processes have a wide and shallow weld-penetration shape such that the welding effect is insufficient. To form a deeply welded metal portion, increasing the number of passes is required. When the number of passes is increased however, the welding efficiency can be adversely lowered.
Other welding methods include MAG (metal-active-gas) weld, MIG (metal-inert-gas) weld, plasma weld, and so on, which are used when deeply welded metal portions and highly efficient welding operations are required. MAG and MIG welds have problems however, including deterioration of welding quality and occurrence of welding defects. Moreover, plasma welds have narrow tolerance ranges for groove accuracy, making the method difficult to use in a construction site or the like.
Methods for improving the problems of shallow welded metal portions that are significant in TIG welding processes have been proposed. These methods utilize a shielding gas, including a mixed gas obtained by mixing an inert gas like argon with hydrogen or helium. Other recently proposed TIG welding methods use active flux (the A-TIG welding method). However, these welding methods which use a shielding gas containing hydrogen are difficult to apply to a material other than austenitic stainless steel because of problems including generation of blowholes and embrittlement of the welded metal portion. In addition, using helium is not preferable in consideration of the cost. Moreover, the workability of the method using active flux is poor, since a coating operation is required before the welding process. Also, since considerable slag is generated on the weld bead, the appearance of the bead is poor, and slag removal is required in multi-layer welds to prevent the degradation of the bead appearance. Furthermore, a lot of fume is generated in the above welding methods which adversely affects the work environment.
In anode-type (DC electrode positive type) arc welding methods including MAG welding, a shielding gas including argon gas mixed with an oxidative component, such as carbon dioxide in a concentration of 20%, is used for improving the stability of the electric arc. However, when an oxidative gas is used as a shielding gas in TIG welding processes, the electrode is easily deteriorated and cannot be used in long-time welds or used repeatedly. If a deteriorated electrode is used, the welding quality is not stable, and weld defects may possibly occur.